Chemical kinetics:how chemical processes proceed over time. Reaction: H 2 (g) + I 2 (g) 2 HI (g) Rate =

Transcription

1 Chaper 13: Chemical kineics Chemical kineics:how chemical processes proceed over ime Reacion rae:change in ime of molar concenraion of reacan or produc of a chemical reacion Influenced by: Concenraion of reacans Temperaure of reacans Surface area of reacans Concenraion of caalys Reacion rae Reacion: H 2 (g) + I 2 (g) 2 HI (g) Rae = Time (s) [H 2 ] (M) = [H 2 ] = Rae = Wha is [HI] over his ime period? Reacion rae is always a posiive value. Calculaing reacion rae for reacion A B Le's say ha from 4 s o 6 s, goes from 2.5 M o 2.3 M [B] goes from.9 M o 1.1 M Rae = ch13blank Page 1 ch13blank Page 2

2 Rae law Rae law: relaes insananeous rae wih concenraions of reacans for his simple reacion: A B he rae law looks like his: rae = k n where k= = n= n= n= 1 n= 2 Mehod of iniial raes Deermining he order of a reacion Reacion: A B Record vs ime wih =.1 M,.2 M, and.4 M Rae = k n 2.79E-3 M/s = k(.2 M) n 1.4E-3 M/s = k(.1 M) n =.4 M iniial rae = 5.58E-3 M/s =.2 M iniial rae = 2.79E-3 M/s =.1 M iniial rae = 1.4E-3 M/s Subsiue any rial's daa ino he rae law o solve for k. ch13blank Page 3 ch13blank Page 4

3 Mehod of iniial raes Reacion order for muliple reacans.5.5 For he reacion a A + b B c C + d D, Zero order Second order (M) Ini Rae (M/s) (M) Ini. Rae (M/s) E E E-3 rae = k m [B] n I - + ClO - OH - IO - + Cl - (ca.) [I - ] [ClO - ] [OH - ] Ini rae (M/s) Exp E-2 Exp E-2 Exp E-2 Exp E-2 rae = k [I - ] m [ClO - ] n [OH - ] p Find pairs of rials where only one concenraion changes o figure ou he orders (m, n, p) of he rae law. ch13blank Page 5 ch13blank Page 6

4 Reacion order for muliple reacans Inegraed rae laws: firs-order If he reacion A B is firs-order, Rae = Using calculus, he differenial equaion can be solved for : ln = k ln = k + ln (which fis he equaion for a sraigh line) Wha is he overall order for his reacion? 5.E-1 4.E-1 3.E-1 2.E-1 1.E-1.E ln y = -.14x ch13blank Page 7 ch13blank Page 8

5 Inegraed rae laws: second- and zero-order If A B is second-order, Rae = 1 1 Inegraed rae law: = k ln / (M -1 ) y =.625x Mah wih inegraed rae laws (equaions will be given on exam) Zero-order: = k + Firs-order: ln 1 1 Second-order: = k + = k or ln = k + ln For he reacion 2 HI(g) H 2 (g) + I 2 (g), he rae law was found experimenally o be rae = k [HI] 2. If k= 2.4 x 1 21 M 1 s 1, and [HI] =.1 M, a wha ime is [HI] =.9 M?.5 If A B is zero-order Rae = Inegraed rae law: y = -.73x +.4 = k Wha is he order and rae consan? Sraigh line Slope -order 1s-order 2nd-order ch13blank Page 9 ch13blank Page 1

6 Reacion half-life Half-life( 1/2 ) = ime required o decrease reacan concenraion by half. Reacion half-life Iodine-123 undergoes firs-order radioacive decay wih half-life of 13.1 hours. Wha is he rae consan, k? For firs-orderreacions, ln To calculae he half-life, = k 1 = 2 1/2.693 (1s order) = k How long will i ake for 99.9% of he iodine-123 o decay? 1/2 1/2 1 (2nd order) = k (zero order) = 2k ch13blank Page 11 ch13blank Page 12

7 Effec of emperaure on rae Temperaure dependence on rae exiss inside k, he rae consan - i's acually only consan a a cerain emp. Arrhenius equaion: k = Ae ( E / RT) a A = frequency facor E a = acivaion energy (barrier) Using he Arrhenius equaion Arrhenius plo: experimenally deermining acivaion energy and frequency facor k = Ae ( E / RT) E a a 1 lnk = lna R + T R= J/mol K T= abs emp, K Svane Arrhenius Swedish chemis Two-poin Arrhenius equaion: for calculaing he acivaion energy by measuring he rae consan a wo emperaures k2 Ea 1 1 ln = k1 R T1 T2 For a cerain reacion, he following rae consans were deermined experimenally: 5. K k M 1 s 1 T 1 T K k M 1 s 1 Wha is he acivaion energy in kj/mol? ch13blank Page 13 ch13blank Page 14

8 Reacion mechanisms Wha acually deermines he order of a reacan? Mos reacions occur as a sequence of several seps. Elemenary sep: a single molecular moion Reacion mechanism:he series of elemenary seps ha make up an overall chemical reacion. Example: slow decomposiion of H 2 O 2 : 1) 2) H 2 O 2 H 2 O + O (slow sep) H 2 O 2 + O H 2 O + O 2 (fas sep) Rae and mechanism Rae laws CANNOTbe prediced for a balanced chemical equaion, since you don' know wha elemenary seps make up is mechanism. Rae laws CANbe prediced for elemenary seps because hey're a single moion -jus use concenraions of reacans. An overall reacion is only as fas as he slowes (rae-deermining) sep of is mechanism (any faser seps will have o wai for he slowes sep) 1) 2) H 2 O 2 H 2 O + O (slow sep) H 2 O 2 + O H 2 O + O 2 (fas sep) Rae = Inermediae: formed in one sep and consumed in anoher - does no appear in overall chemical equaion I The rae law for 2 H 2 O 2 2 H 2 O + O 2 is firs order in boh ca peroxide and iodide. Which is a plausible mechanism? 1) 2) 1) 2) H 2 O 2 + I IO + H 2 O H 2 O 2 + IO I + H 2 O + O 2 H 2 O 2 H 2 O + O H 2 O 2 + O H 2 O+ O 2 ch13blank Page 15 ch13blank Page 16

9 Caalysis A caalysis consumed in an early mechanisic sep, and hen re-formed in a laer sep -meaning i should never be consumed overall. Caalyss increase reacion rae by providing an alernae mechanism wih lower acivaion energies. ch13blank Page 17